Since I'm thinking small (and no new holes in the walls), I looked around and found this.
Amazon.com: Bionaire BAP336-U EverFresh Air Exchange System: Kitchen & Dining
I'm not sure how this works, and the from factor is 'out there',
but it shows that my idea of a DIY window mounted unit, wasn't very original.
But that's not going to stop me from trying to figure out how to make one..